554 
NATURE 
| Aprel 12, 1883 
deflection at the end of 5 minutes was 460° on the scale 
(see accompanying table). This period of time was 
adopted for this reason, for the duration of each following 
experiment, though more than needed to produce maxi- 
mum results. So far the evidence is conclusive that dark 
heat (¢.e. heat capable of melting ice) applied to one side 
of a piece of ice does ot affect the thermopile on the 
opposite side. So much for the negative results. 
It seemed to me at this point worth while to investigate 
the effects produced by /wmznous radiant energy of various 
phases of quality after transmission through ice, which, 
it would appear, effectually barred the passage of all the 
obscure rays of the iron ball from even entering it, while 
the liquefaction of the ice at the surface was beyond all 
comparison greater than that which goes on at the surface 
of a glacier even with a full midsummer sun. The 
sources of /usinous energy chosen are given in the first 
column of the following table. The feeble effect produced 
by the blue flame of a very large Bunsen lamp (giving no 
red, orange, or yellow when examined with the spectro- 
scope) as compared with the effects produced by the more 
highly luminous gas-flames of far inferior thermal in- 
tensity (which gave, of course, a complete visible spec- 
trum), is extremely interesting for the light it throws 
upon the subject in hand. The table of results explains 
itself at once to any student of physics. The lime-light 
used, it may be added, was a very powerful one; the 
sunshine, however, was not very bright or very constant, 
owing to the drifting of clouds. The latter fact explains 
the apparent slight anomaly in the results of the solar 
radiation given in Series I]. and III. The observations 
were made however with the solar radiation (as esti- 
mated by a Crookes’ Radiometer) approximately the same 
for them all, 
Tab. lo show the Sifting Power of Ice and Snow upon Radiation 
of different Phases of Quality 
Ser. 111. Ser. 1V. 
° ith it - , 
Series eel neh ofl ictal ee B 
Sources of radiant Siena inch of | leat ice | ROnGCS sich 
relative (with ((asinIII.) 
Ges radiant |Vcty, clear many air| with fallen 
energy. |'° 1MtEI-| bubbles) |much we | SM2W 17 
; oe inter-. | snow on | t€*Posed- 
posed, | one side. 
1. Red-hot iron ball, 
3 inches diameter 
(at dull red heat). | 460°00) 0709, o'cO | oO'cO | O00 
2. Large Bunsen 
lamp flame (feeble 
luminosity) giving 
incomplete spec- | 
frm! ee eee ees. eI'35COl|)  2500)|| 2:00) | OfOOmIG:co 
3. Small Bunsen | 
lamp flame, with } 
air shut off below | 
(giving complete 
SPecteiny)) eee) snes] 777.00 6°00, 400} 200] 000 
4. Small fish-tail gas- 
burner... A 87°00) 12°00 700 | 6'00 o'co 
5. Lime-light... 192700 | 51°20) 38°40 | 20°48 0°00 
6. Lhe Sun 530°00 | 310°00 , 320:00!}  — 13 00 
The numbers in each series in the foregoing table do not 
give very simple relations among themselves, and each 
number must be regarded as only a near approximation 
to the exact truth. Still, when all those slight inaccuracies 
which ari-e from “errors of experiment ’’ are allowed for, 
the general meaning and bearing of the facts remain, 
namely, that though heat (gvd@ heat capable as it is of 
melting ice) cannot enter ice, yet wminous energy, which 
7s readily absorbed and transformed into heat by opaque 
* In this case a 4-inch plate of clear ice was used. 
and semt-opaque bodies, can enter and pass through the 
ice, until it meets with a non-transparent body. Substi- 
tuting for our thermopile in the experiment, stones, dirt, 
organic germs, &c., within the glacier, we at once per- 
ceive how the luminous radiant energy of the sun can (by 
being transformed into dark heat) play its part in pro- 
ducing the movement of glaciers. 
Further, this will be found, I believe, the ov/y satis- 
factory explanation yet given of the remarkable facts (1) 
that glaciers move faster (in the Alps about twice as fast; 
during the summer than during the winter; (2) that the 
motion during the day is greater than during the night. 
This fact most people who have written on glaciers have 
found it difficult to explain, for when the “ Regelation 
Theory” is fully accepted, and all that follows from it is 
recognised, and when due allowance is made for z¢ernalZ 
Jriction, we still must seek for a cause, independent of 
both of these, to account for the varéations in the move- 
ments of glaciers, day and night, summer and winter, 
This cause has now, I think, no longer to be sought for. 
The glacier may be compared to a large greenhouse ; 
as luminous energy enters freely through the glass in the 
one case, so it enters freely through the transparent ice 
in the other ; in both cases, heat available for work is 
produced by its transformation. 
In the glacier this work is expended in diminishing 
the cohesion of the molecules of those parts of the ice 
which are in contact with the bodies which absorb the 
luminous energy. The beautiful silvery blue light of an 
ice-cavern seems to show that a part of a beam of lumi- 
nous radiation is absorbed by clear ice. 
The Series IV. and V. of the table illustrate the effect 
of (a) the more or less granular condition of the ice in 
many parts of a glacier, (4) the snow with which the 
glaciers are covered during the winter. The diffusive 
action of the latter upon luminous energy is seen by 
reference to Series V. to be very great; hence the 
necessity for the use of coloured spectacles on the higher 
glacier regions. A. IRVING 
DEDUCTIVE BIOLOGY 
T has probably occurred to a good many readers of 
NATURE that it would be well if some one were to 
utter a word of warning as to the mischief which may be 
done, and especially to students, by the present fashion 
of explaining all kinds of complicated morphological 
phenomena in a more or less purely deductive fashion. 
Itis no doubt pleasant, even fascinating, to sit down at 
one’s desk and, having formulated a few fundamental 
assumptions, to spin out from these explanations of what 
we see in the world about us. But I think when done it 
should be understood that the result is merely a literary 
performance, and though, viewed in that aspect, one may 
admire the skill and neatness with which it is accom- 
plished, I nevertheless venture to think that the whole 
proceeding is harmful. 
Now,as I shall attempt to illustrate my position by refer- 
ence to papers which have appeared in NATURE in parti- 
cular, I may as well say at once that [ have no personal or 
merely controversial object in writing these lines, But 
though it is nowno part of the business of my life to take part 
in teaching, Ihave had some experience of it, and a great 
deal too much of testing its results by the process of 
examination. I have derived then a tolerably definite 
idea—as I believe—of the difficulties that beset the im- 
parting of scientific instruction, and a decided conviction 
as to what sort of discipline is wholesome, and what is 
mischievous. 
Of course I do not deny—far from it—the inspiriting 
influence which large generalisations impart to teaching. 
But then I think the intellectual enjoyment of them must 
be earned. The first thing to do is to put before the 
| student the facts, and then, when these are conscientiously 
